The category /transport is a primary parent category containing data and methodologies for calculating greenhouse gas emissions associated with transport. The data within this category is taken from several sources including The World Resources Institute Greenhouse Gas Protocol, the UK Government agency DEFRA and the EMEP/CORINAIR Emission Inventory Guidebook - 2007.

The /transport category is organised primarily according to transport type (i.e. vehicle type) and the nature of the emissions factors which form the basis of calculations which are described below.

'Generic' versus 'specific' datasets

Within most categories a distinction is made on the basis of data and methodologies which provide generic emissions calculations and those which provide calculations relating to more specific scenarios. The term 'generic' here refers to data and associated calculation methods which are based on generalised, representative types of transport, for example 'small, diesel car', 'local train', or 'long-haul flight'.

'Specific' datasets use actual fuel consumption and greenhouse gas emissions information for particular models of vehicle. Examples might be emissions calculations for car travel in a 3-door, 1.4L Volkswagen Golf, or a flight in an Airbus A318. These datasets provide more accurate emissions estimates for a given transport scenario but require more detailed information from the user.

'Generic' and 'specific' data and methodologies are typically organised into distinct subcategories within each respective transport category. Generic subcategories are present in nearly all of the main transport categories, with alternative datasets from differing sources available in some cases. Specific datasets are currently available in the transport/car/specific category (containing 9700 UK and 1151 US car models), and the transport/plane/specific category (containing over 140 jet, turboprop and military aircraft types).

Differences in emissions factor type

A further distinction between datasets contained within the transport category is the precise form of the emissions factors used. The following types of emissions factor are utilised throughout the transport categories, with each providing a slightly different type of emissions estimate and requiring particular inputs from the user:

by fuel

The most direct way to measure transport-related greenhouse gas emissions, is to use the quantity of fuel consumed, if it is known. As such, Discover contains transport-associated greenhouse gas emissions factors which relate emissions with specific quantities of fuel, e.g. kg CO2 per litre. Such data can be found for numerous types of fuel in the category /transport/by fuel, and requires the user to simply specify a quantity of fuel consumed in order to obtain an emissions estimate.

by distance only

The most common type of transport-related greenhouse gas emissions factor specifies a quantity of emissions in terms of a distance travelled, for example, kg CO2 per km. Such emissions factors are dependent, to a greater or lesser extent, on the fuel consumption properties of vehicles and implicitly represent emissions on a per vehicle basis (which can be contrasted with per passenger emissions factors, described below). In these cases the user is simply required to specify a distance travelled (as well as the transport type) in order to obtain an emissions estimate. Such emissions factors are used in several transport categories, particularly those which feature private vehicles (e.g. car, motorcycle, van).

by distance, by passenger

In many cases, greenhouse gas emissions data are specified on a per passenger basis, e.g. kg CO2 per km per passenger. This type of factor is typically provided for public/passenger modes of transport and usually incorporate assumptions regarding the typical occupancy of the vehicle, allowing the total vehicle emissions to be 'shared' amongst passengers. Such emissions factors are used in several transport categories, particularly those featuring public modes of transport (e.g. bus, plane, ship, train), and require the user to specify a distance travelled and the number of passengers being accounted for (although the latter usually defaults to 1 if not specified).

by distance, by mass

Where freight is transported, associated greenhouse gas emissions are related to both the distance travelled and the mass of freight involved. As such, emissions factors for freighting take the form of a quantity of greenhouse gases emitted per distance per mass, e.g. kg CO2 per km per tonne. Such freighting-related methodologies can be found within the transport category under several of the vehicular subcategories (e.g. large goods vehicle, plane, ship, train) in discrete /freight subcategories (e.g. transport/plane/generic/freight). In these cases, the user is required to provide both distance and mass quantities in order to calculate their greenhouse gas emissions.

Additional functionality

It is the aim of Discover to provide the most useful and flexible emissions calculations for users. With this in mind, several categories within the transport category offer functionality which mimics the choices offered by different emissions factor types. For example, in those categories which use per vehicle emissions factors (e.g. car, large goods vehicles, motorcycle, van, and one subcategory of the bus category) users may specify the occupancy of the vehicle so that the total vehicular emissions may be converted into per passenger emissions if desired. (Note that it is usually not possible to do the converse - i.e. calculate vehicular emissions from original per passenger emissions factors - as the assumed occupancies on which the per passenger emissions factors are based are not available in most sources).

Similarly, categories which feature 'private' vehicles (car, large goods vehicles, motorcycle, van) also include the facility to specify vehicle fuel consumption, if known. In these cases, the vehicle fuel consumption data is combined with data on the carbon intensity of the respective fuel to derive a more accurate distance-based emissions factor.

To learn more about the data, calculation methodologies and functional options available within the various transport categories, consult the appropriate documentation pages.